The present invention relates to semiconductor devices, and more particularly to a method and system for determining the operating voltage for a semiconductor device.
FIG. 1 depicts a conventional semiconductor device 10. The semiconductor device 10 includes active areas 11 and 12 where devices, such as memory cells and/or logic, are formed. Across the active areas 11 and 12 are conductive lines 14 and 16. The conductive lines 14 and 16 are preferably polysilicon lines. The semiconductor device 10 also conventional silicon trench isolation (xe2x80x9cSTIxe2x80x9d) structures 18 between the active areas 11 and 12. The STI isolation structures 18 are used to isolate different portions of a semiconductor device. Although only some of the conventional STI structures 18 are marked, there are additional conventional STI structures. Furthermore, the conventional semiconductor device 10 typically includes other devices (not shown).
FIG. 2 depicts a device 20 formed in the conventional semiconductor device 10. The conventional semiconductor device 10 could include other devices, such as, memory cells. The device 20 includes a gate 22 having spacers 24 and 26. The spacers 24 and 26 are typically between five hundred and one thousand Angstroms in thickness. The gate 22 is separated from the underlying substrate 21 using oxide 23. The device 20 also includes a source 30 and a drain 34. The source 30 also includes a source extension 34 that is under the gate 22. Similarly, the drain 32 includes a drain extension 36 that is under the gate 22. The source extension 34 and the drain extension 36 are typically between eighty and one hundred Angstroms in thickness. Also depicted are conventional STI structures 40 and 42 that isolate the device 20 from other portions of the semiconductor device 10.
In order to operate the conventional semiconductor device 10, an operating voltage must be selected. In order to choose the operating voltage, the maximum operating voltage allowed to be used with the conventional semiconductor device 10 is selected. In order to do so, the lifetime of the conventional semiconductor device is determined. Typically the lifetime is determined using a time dependent dielectric breakdown (xe2x80x9cTDDBxe2x80x9d) test and/or a voltage ramp dielectric breakdown (xe2x80x9cVRDBxe2x80x9d) test on a particular conventional semiconductor device 10. The TDDB test applies a particular voltage to the conventional semiconductor device 10 until the conventional semiconductor device 10 fails. The VRDB test applies an increasing voltage, typically one that increases in steps, to the conventional semiconductor device 10 until the conventional semiconductor device 10 fails. Thus, the lifetime of the conventional semiconductor device 10, including the dependence of the lifetime on the operating voltage, can be determined.
Based on the lifetime experimentally determined and the desired lifetime for the semiconductor device 10, the maximum operating voltage of the conventional semiconductor device 10 is determined. During operation, an operating voltage that is less than or equal to the maximum operating voltage is utilized. As a result, the conventional semiconductor device 10 should last for the desired amount of time. For example, it is typically desired to have a lifetime of ten years during use. The operating voltage used and the maximum operating voltage allowed to be used with the conventional semiconductor device 10 are set so that the lifetime of the conventional semiconductor device 10 is as desired.
Although the conventional semiconductor device 10 functions, one of ordinary skill in the art will readily realize that the polysilicon lines 14, 16 and 22 affect the lifetime at a particular operating voltage. In particular, the source extension 34 and the drain extension 36 can result in a weaker oxide 23. In addition, the areas of the source extension 34 and the drain extension 36 are sites for a low voltage leakage current, even for the off state of the device 20. Moreover, the effect of the leakage current increases as the length of the channel is decreased when the gate 22 is made less wide. Thus, as the conventional semiconductor device is scaled down to allow for a higher density of devices 20, problems due to leakage current, as well as problems with the quality of the oxide 23, increase. Consequently, it would be desirable to account for the overlap between the gate 22 and the source 30 and drain 34 in the area of the source extension 34 and the drain extension 36 could be accounted for.
Furthermore, the STI structures 18, 40 and 42 can reduce the lifetime of the device. FIG. 2B depicts a conventional STI structure 42. However, the other conventional STI structures in the conventional semiconductor device 10 may suffer from the same defects. The conventional STI structure 42 includes conventional trench 44, which is filled with conventional oxide filler 46. Near the corners of the conventional STI structure 42, the oxide filler 46 has thinned in areas 48 and 50. The thinned areas 48 and 50 reduce the ability of the STI structures 18, 40 and 42 to insulate the devices 20. As a result, a leakage current can occur through the thinned areas 18, 40 and 42. The leakage current can lower the threshold voltage of devices fabricated near the conventional STI structures 18, 40 and 42, which adversely affect performance of the conventional semiconductor device 10.
The thinned areas 48 and 50 may occur for a variety of reasons. Typically, silicon wafers having a (100) orientation (shown in FIG. 2B) are used for fabricating conventional semiconductor devices 10. Because the top surface has a (100) orientation, near the corners of the trenches 48 and 50, the exposed silicon has a (111) orientation. The (111) orientation of silicon has a larger number of dangling bonds. Thus, when the oxide filler 46 is provided, areas near the (111) orientation are thinner. In addition, mechanical stress tends to concentrate at areas where a corner is fabricated. Mechanical stress also tends to cause a thinning of the oxide filler 46 near the corners of the conventional STI structures 18, 40 and 42. In addition, as discussed above, in more recent conventional Flash memory devices, a nitride oxide, such as N2O is used in forming the gate oxide for the memory cells in the core region. When N2O is used, the thinning that results in the areas 48 and 50 is even more severe. Thus, the problems due to leakage current in the semiconductor device 10 are made worse.
Accordingly, what is needed is a system and method for determining the operating voltage of the semiconductor device that takes into account the overlap between the source and/or drain extensions and the polysilicon lines as well as the STI structures. The present invention addresses such a need.
The present invention provides a method and system for determining an operating voltage for a semiconductor device. The semiconductor device includes at least one active area, at least one silicon trench isolation (STI) structure and a plurality of polysilicon lines. The method and system comprise determining a first plurality of lifetimes and a second plurality of lifetimes. The first plurality of lifetimes is determined for a first plurality of semiconductor devices having a first plurality of polysilicon lines, at least a first active area and a first plurality of STI structures for separating the at least the first active area. The first plurality of polysilicon lines has a particular area and a plurality of peripheral lengths. Each of the first plurality of STI structures has a length. The second plurality of lifetimes is determined for a second plurality of semiconductor devices having a second plurality of polysilicon lines, at least a second active area and a second plurality of STI structures for separating the at least the second active area The second plurality of polysilicon lines has a plurality of areas and the particular peripheral length. Each of the second plurality of STI structures has the length. The method and system also comprise determining the operating voltage based on the first plurality of lifetimes and the second plurality of lifetimes.
According to the system and method disclosed herein, the present invention provides a method and system for determining the maximum operating voltage for a semiconductor device that takes into account the effects of STI structures separate from the effects of polysilicon lines.